Supercritical carbon dioxide | Simple Closed Brayton Cycle
Objectives: Demonstrate initial cycle performance with reduced risk configuration Single compressor configuration Turbine inlet near 500°C similar to WHR applications Provides steady & transient cycle performance data used to predict RCBC performance and operation
Recompression Closed Brayton Cycle
Objectives: Demonstrate high performance cycle with parallel compressors and multiple heat exchangers Increase turbine inlet temperature to 715°C Measure steady & transient cycle performance data, evaluate operability Demonstrate pathway to 50% thermal efficiency
Above: Two sCO2
The turbine casing was fabricated from IN 625 for cost and schedule reasons, as well as due to the unfavourable experiences with HA282 casting for both the turbine casing in the predecessor SunShot project and the STEP Demo turbine control valve. A fabricated 740H casing was originally planned, but IN 625 was less costly. Use of IN 625 comes with a reduced embrittlement life limit, but is well within the expected life of the STEP project. Although IN 625 is a well-known alloy, weld issues were encountered in the casing welds, but due to the case thickness and physical access, radiographic testing inspection was not possible for the interior welds. The issue was resolved for simple cycle operation with a combination of analysis and proof testing. Fracture mechanics analysis and additional material testing were conducted to accept the casing for use up to simple cycle conditions (500°C) and further testing is planned to assess 715°C service. The compressors, both main and bypass, leverage the existing Baker Hughes commercial product line as well as work undertaken as part of a US Department of Energy funded project under the Apollo concentrating solar power programme.
The compressor rotors employ a monolithic design for the reduced size impellers inherent in the compact sCO2
power cycle. They include inlet
cycles to be demonstrated at the STEP Demo facility
guide vanes with large angle variation to support operational flexibility. The main compressor is capable of low-speed start under liquid or two- phase conditions. This enables restart with liquid or dense phase CO2
in the loop, which can occur
during extended pressurised holds as the CO2 in the loop approaches ambient temperatures.
Significant analysis was conducted between Baker Hughes and the project team to confirm operation over the test requirements to ensure equipment integrity and surge control. Significant performance differences in factory acceptance testing vs site testing are still being resolved.
The project team
The project team is strong and motivated, working hard to complete facility construction and testing. It has a nice mix of highly trained and experienced staff specialised in both the required component technologies and in plant construction and operations, and younger staff able and motivated to advance these technologies well beyond the life of the project. The team has experienced many changes in the last seven years. It has however been anchored by consistent key leadership and commitment at GTI Energy but with some turnover of principal investigators (PI)/leaders. At SwRI there has been consistent organisational commitment and broad
18 | November/December 2023|
www.modernpowersystems.com sCO2
operating experience, but changes in PI and leadership changes, while GE Research has also experienced changes in PIs and leadership. The DOE as well has experienced changes in programme leadership over three presidential administrations. Throughout these changes, the project remains on-track and is expected to provide critical support to the commercialisation of sCO2
power technology. Next steps
The project’s next steps are to finalise system commissioning and conduct simple cycle testing up to 500°C TiT in early 2024. The team aims to secure industry cost share and DOE approval to proceed into the final budget period 3, which will reconfigure the pilot plant to the Recompression Closed Brayton Cycle configuration and complete testing up to the full 715°C operating potential of the plant. This is expected to extend through 2025. In conclusion, supercritical carbon dioxide power cycles hold great potential to improve efficiency, reduce the size of future power plants, reduce water usage, and lower CO2
emissions.
The STEP Demo project will help lay the groundwork for wider deployment of sCO2
power
cycles and that means cleaner, more efficient, and less expensive power generation. Follow progress at
www.STEPdemo.us
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